Minbyun claimed there is a legal problem because the ministry has failed to do the things it is supposed to do as the subject of the THAAD system's deployment, including the announcement of the project of deploying the THAAD system and the appraisal of strategic environmental effects.

"The deployment of THAAD, an ancillary facility held by foreign troops stationed in our country, is viewed as a defense and military project," Minbyun said, adding the law on defense and military projects stipulates that the defense minister should give public notice about the project.

The ministry, however, has maintained that the THAAD deployment is not subject to South Korea's defense and military facility project law or the law on environmental effects because it is part of the US army's projects based on the Status of Forces Agreement, which defines areas of legal responsibility of the 28,000-strong US soldiers in South Korea.

Minbyun said it has confirmed the illegality of the project of the THAAD deployment, vowing to deal with any further legal problems related with it.

On Tuesday, South Korea's military formally signed a land-swap deal with Lotte Group, paving the way for the installation of an advanced US missile defense system this year as scheduled, the defense ministry said.

The Ministry of National Defense and Lotte Group have agreed to exchange properties to complete the deployment "within this year," the ministry said in a statement.

The anti-missile system is widely expected to be installed in June or July. Pyongyang's continued provocations apparently provided a strong impetus for Seoul and Washington to agree in July last year to deploy the THAAD system in South Korea by the end of 2017.

In early November, two months after the North's fifth nuclear test, US Forces Korea Commander Gen. Vincent Brooks said a THAAD battery will "come in the next eight to 10 months." (Yonhap)